FIG. 14 indicates an example of the construction of a prior art FM stereo receiving device, in which reference numeral 1 is a receiving antenna; 2 is a frequency converter; 3 is a local oscillator; 4 is a limiter, 5 is a discriminator; 6 is a band pass filter for fp=19 kHz (pilot signal); 7 is an oscillator for 2 fp=38 kHz (subcarrier); 8 is a band pass filter having a pass band of 23 to 53 kHz; 9 is a low pass filter having a cut-off frequency of 15 kHz; 10 is a synchronous detecting circuit; 11 is a low pass filter having a cut-off frequency of 15 kHz; 12 is a matrix amplifier; 13 and 14 are de-emphasis circuits; 15 and 16 are ER and EL output terminals, respectively.
In FIG. 14, a signal received through the antenna 1 is applied to the frequency converter 2 to be converted into an IF signal having an IF (intermediate frequency) of 10.7 MHz, which is a difference frequency between the received signal and a signal coming from the local oscillator 3. The amplitude of this IF signal is limited by the limiter 4 and it is applied to the discriminator 5, where it is subjected to a conversion of frequency of FM-modulated wave-voltage to obtain a baseband signal. This baseband signal is applied to the band pass filter 6, from which a pilot signal of fp=19 kHz is taken out. The oscillator 7 starts a coupling oscillation by this signal.
On the other hand, the baseband signal described above is applied to the band pass filter 8 and a low pass filter 9 and a stereo difference signal EScos2.omega.pt having frequencies of 23 to 53 kHz taken out from the band pass filter 8 is sent to the synchronous detecting circuit 10. The synchronous detecting circuit 10 synchronous-detects the difference signal described above on the basis of an output signal cos2.omega.pt of the oscillator 7 described above. A stereo signal ES is obtained by taking out the detected output thus obtained through the low pass filter 11.
Further the low pass filter 9 outputs also a monaural signal EM. This monaural signal EM is applied to the matrix amplifier 12 together with the stereo signal ES described above. The signals ER and EL before the emphasis are obtained from these signals ES and EM. The right ear signal ER and the left ear signal EL are obtained by making these signals pass through the de-emphasis circuits 13 and 14.
FIG. 13 is a characteristic scheme indicating a baseband signal and a demodulation noise distribution in an FM broadcasted wave. As it can be understood from the frequency distribution of the baseband signal in the FM broadcasted wave indicated in the figure, when it is FM-demodulated, noise in the middle course has a distribution proportional to the baseband frequency indicated by a broken line in FIG. 13. For this reason, by the demodulating system indicated in FIG. 14 described above, since the SN ratio of the difference signal component is lowered by about 20 dB with respect to that of the monaural signal component, the effective reception region for the FM broadcasted wave is narrowed equivalently.
FIG. 15 shows another example of a prior art FM stereo receiver, in which 31 is an antenna; 32 is a local oscillator; 33 is a frequency converter; 34 is a band pass filter having a central frequency of 10.7 MHz; 35 is a limiter; 36 is a frequency detecting circuit (discriminator); 37 is a band pass filter having a pass band of 19 kHz; 38 is a subcarrier generating circuit; 39 is a band eliminating filter having a frequency eliminating band of 19 kHz; 40 is an electronic switch; 41 and 42 are low pass filters having a pass band of 0-15 kHz; 43 and 44 are de-emphasis circuits; 45 is a matrix circuit; 46 is a right output ER; and 47 is a left output EL; 40 to 45 constituting a stereo demodulating section 18.
Hereinbelow the operation of the prior art FM stereo receiver described above, will be explained. A high frequency signal received by the antenna 31 is applied to the frequency converter 33 together with an output of the local oscillator 32 to be converted in the frequency. An IF signal is obtained by making the output thereof pass through the band pass filter 34. This IF signal of FM modulated wave is applied to the discriminator with an amplitude kept constant by the limiter 35 and in this way the baseband signal EB given by Equation (1) is reproduced.
The baseband signal EB of stereo broadcasted wave can be expressed by Equation (16) EQU EB=EM+ES cos 2 .omega.pt+P cos .omega.pt (16) EQU where EM (monaural signal)=1/2(ER+EL) (17) EQU ES (stereo signal)=1/2(ER-EL) (18)
and Pcos.omega.pt:pilot signal
The frequency distribution of the baseband signal EB is indicated in FIG. 14, in which a stereo modulated wave of 38.+-.15 kHz using a component of 38 kHz (2 fp) as a subcarrier is superposed on the monaural signal EM of 0-15 kHz and the pilot signal of fp=15 kHz is inserted therebetween.
The pilot signal fp=19 kHz expressed by the third term in Equation (16) is taken out by making this baseband signal EB pass through the band pass filter 37 and the subcarrier is reproduced by the subcarrier generating circuit 38 on the basis of this signal.
This subcarrier can be obtained either by squaring the pilot signal described above or by phase coupling of the oscillator of frequency 2 fp. By the first method, by squaring the pilot signal Pcos.omega.pt EQU P.sup.2 cos.sup.2 .omega.pt=P.sup.2 /2[1+ cos 2.omega.pt] (19)
is obtained and the component of the second term in Equation (19) is taken out through a band pass filter having a pass band of 38 kHz.
On the other hand, the output of the discriminator 36 is made pass through the band eliminating filter 39 for eliminating only the component expressed by the third term in Equation (16) and a signal EB' given by Equation (20) is obtained. EQU EB'=EM+ES cos 2 .omega.pt (20)
The signal EB' is applied to the electronic switch 40 and switched by driving it by cos 2 .cndot.pt=1 and cos 2 .omega.pt=-1. In this way a signal having a waveform, in which the right output ER and the left output EL are switched at cos2.omega.pt=1 and cos2.omega.pt=-1, respectively, is obtained. However, at this time, since the magnitude of ES is multiplied theoretically only by 2/.pi., it is compensated later by the matrix circuit 45.
The output of the electronic switch 40 is applied to the matrix circuit 45 through the low pass filters 41 and 42 as well as the de-emphasis circuits 43 and 45 to compensate the insufficient part (difference component) of ES described above, in order to obtain the ER and EL outputs.
However also the prior art FM stereo receiver described above has a following problem. That is, when noise having a flat frequency distribution is added to the received signal, the FM demodulated output of the discriminator 6 includes so-called triangular noise proportional to the frequency indicated by a broken line in FIG. 13. That is, supposing that the noise En added to the IF band before the FM demodulation is given by: EQU En=n cos .omega.nt (21)
and that the central frequency of the IF band is .omega.i' (=.omega.i+.omega.O.sub.2), the noise e.sub.n after the FM demodulation corresponding to that given by Equation (21) is expressed by Equation (22). EQU e.sub.n n(.omega.n-.omega.i') sin(.omega.n-.omega.i')t (22)
Putting .omega.n-.omega.i'=.omega.n' in the expression of the baseband, Equation (22) is transformed into Equation (23). EQU e.sub.n =n.omega.n' sin .omega.n't (23)
Equation (23) expresses so-called triangular noise, which has a distribution indicated by the broken line in FIG. 13. Therefore it had a problem that noise in the neighborhood of .omega..apprxeq.2.omega.p is transformed into a low frequency at the stereo demodulation and lowers the SN ratio for the stereo signal ES. That is, when this noise is switched by means of the electronic switch 40, it is transformed into noise below 15 kHz and in this way the SN ratio is lowered with respect to that observed in the case where only the EM component is taken out from the output of the band eliminating filter.